I'm trying to install a bathroom off my garage. I have a 4" pvc tee'd into the main drain and want to have three 2" drains tied into it. The roof vent is one foot downgrade from the toilet, and then the shower and sink will be four and eight feet upgrade from that. Do I have to run a vent from the furthest sink drain up into the attic and then tie back into that roof vent? I don't see why I would need to, but my nieghbor seems to think if I don't, the sink drain will run slow. I can't wrap my head around how a half inch water supply line can fill a four inch drain to cause it to drain slow. What am I missing? Thanks in advance for your advice.